ASCIDIANS OF THE BERMUDAS The ascidians of

ASCIDIANS
OF
THE
BERMUDAS
N. J. BERRILL
MCGILL UNIVERSITY,MONTREAL
The ascidians of the Bermuda islands have already been described
by Van Name (1902) and a revision published by the same author in
1921 under the title “¿The
Ascidian Fauna of the West Indies.― In
view of this the present paper is confined to a brief account of the
fauna as a whole, a description of several new species, and a few notes
upon certain other forms.
The ascidians of the Bermudas are almost identical, species for
species, with those of the West Indies and Gulf of Mexico, and there
is little doubt that, together with the bulk of the Bermuda fauna,
they have been carried there from those regions by the west Atlantic
drift.
Like ascidians throughout
the world, their habitat is determined
as a compromise between two factors, their ability to withstand wave
action and exposure, and their need for a good flow of well oxygenated
water.
They may accordingly
be divided into three groups, those
to be found only under stones, those attached to the upper surfaces of
stones, or to various corals and sea-fans, and those found beyond the
outer reefs.
Apart from such classification of habitat further description such
as specified localities is practically unnecessary.
Species found under
stones in one locality are likely to be found in similar positions through
out the islands.
The same holds true for forms that are attached to
exposed surfaces.
The material upon which this paper is based was obtained through
the facilities of the Bermuda Biological Station, St. Georges, and also
through the assistance of LOuis Mowbray, Esq., of the Government
Aquarium.
FAUNA
LIST
Aplidium (Amaroucium) bermudae (Van Name)
Aplidium (Amaroucium) exile (Van Name)
Tri4idemnum savignii (Herdman)
Trididemnum orbiculatum (Van Name)
Didemnum candidum Savigny
Didemnum (Polysyncraton) amethysteum (Van Name)
Leptoclinum macdonaldi (Herdman)
Lissoclinum fragile (Van Name)
Polycitor (Eudisknna) olivaceus (Van Name)
77
on corals
under stones
on algae, under stones
under stones
under stones, on corals
under stones
on corals, etc.
under stones
on stones, corals, etc.
78
N. J. BERRILL
Polycitor (Eudistoma) convexus (Van Name)
Polycitor (Eudistoma) clarus (Van Name)
Polycitor (Eudistoma) capsulatum (Van Name)
Clavelina picta (Verrill)
Clavelina oblonga Herdman
Cystodytes dellechiaiae (Della Valle)
Distaplia (Holozoa) bermudensis (Van Name)
Perophora viridis Verrill
Perophora bermudensis n.sp
Ecteinasci4ia turbinata Herdman
Ecteinasci4ia conklini n.sp
Ecteinascidia conklini var. minuta
Ascidia nigra Savigny
Asci4ia curvata Traustedt
Botryllus schlosseri (Pallas)
Botrylloides niger (Herdman)
Symplegma viride Herdman
Polyandrocarpa (Eusynstyela) tincta (Van Name)
Polycarpa obtecta Traustedt
Styela partita (Stimpson)
Pyura vitlata (Stimpson)
Microcosmus exasperatus Heller
on corals, etc.
on stones, corals, etc.
on corals, etc.
on sea-fans, wrecks, etc.
under stones, on corals
on corals, stones, etc.
under stones, on corals
under stones
under stones
on stones, corals, wrecks
under stones
under stones
on stones, etc.
under stones
under stones
on and under stones
under stones
under stones
on stones, corals, reefs
on stones, corals, reefs
on stones
under stones, on reefs
The Bermuda fauna is exceptionally
rich in members of the family
Perophoridm.
Two well-known
forms, Ecteinascidia
turbinata
and
Perophora viridis, are common.
Their close relationship,
however, is
emphasized by the discovery of transitional
types.
These new forms
may frequently
be found growing under the same boulder as those
two species.
Inasmuch
of immature
as the existing descriptions
zooids, a somewhat
detailed
species; such an account
other forms.
also forms
of Ecteinascidia
turbinata
are
account
is made here of this
a basis for comparison
of the
Ecteinascidia turbinata Herdman.
This species was originally described by Herdman from material
collected at Bermuda by the Challenger expedition.
Large orange
colonies develop attached to the upper surfaces of rocks where currents
are relatively strong.
The essential organization
of the zooid is shown in Fig. 1. The
structures shown here for the first time are the heart, and the gonads
with their ducts.
The heart is of the same relative size and length,
and is in the same relative position as in species of Ascidia or Phallusia.
The gonads develop in the intestinal ioop on the left side, as in Pero
phora,
the
ovary
being
surrounded
by
a ring
of testicular
lobes.
The vas deferens during its distal course follows the intestine and
they open together some distance short of the atrial siphon.
The
oviduct, on the other hand, is relatively short and wide.
It passes
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ASCIDIANS OF BERMUDAS
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N. J. BERRILL
from the ovary to the dorsal lamina, opening beyond it into the atriäl
cavity of the right side. A single egg at a time is forced up the duct.
During this brief passage it becomes greatly elongated
thrOugh
compression,
rapidly
regaining
its spherical
shape
as it emerges.
The above features are generic, the characters of specific importance
being as follows,—the tendency to grow on the exposed surfaces of
rocks, the red or orange colour, the proximity of the atrial to the
branchial siphon, the possession by the adult zoöid of about thirty
rows of stigmata,
and the formation of tadpole larvae with twelve
rows of stigmata.
The breeding season extends from about June first
to August or September.
Ecteinascidia
conklini
n. sp.
This species was at first mistaken for young colonies of E. turbinata.
The generic features are identical with those of E. turbinata.
The
specific
differences
are
the
following:
it is found
side of rocks, the colour is green or yellow-green
ring
around
the
siphons,
sometimes
absent;
the
only
on
the
under
with but a thin red
atrial
siphon
is long
and placed some considerable
distance from the branchial siphon;
the rows of stigmata in the zoöids average about twenty in number,
while smaller tadpoles are formed with but six rows of stigmata;
the eggs are smaller, and the breeding season commences about one
month
later
than
Ecteinasci4ia
that
conklini
of E. turbinata,
var. minuta
under
identical
conditions.
n. sp.
This form was found with mixed colonies of E. conklini typica and
Perophora viridis.
It differs from the type species in a few minor
features only, which nevertheless seem worthy of recording.
It is smaller; the number of rows of stigmata in the zooid rarely
exceeds fifteen; the atrial siphon is short and close to the branchial
siphon while both bear prominent ridges, and the eggs and larv2e are
smaller
although
other.
The breeding
E. conklini
in the former,
Perophora
possesses
six
rows
of stigmata
season as far as is known coincides
the most definite
and minuta
E. turbinata
tadpole
as in the
with that of
typica.
Probably
typica
the
from
tending
is but very
bermudensis
feature
E. turbinata
distinguishing
is the curvature
to form a secondary
slightly
curved
loop.
both E. conklini
of the intestine
The intestine
of
in comparison.
n. sp.
This species forms mixed colonies with the common Perophora viridis,
both growing in great profusion on the under surface of stones where
there is a good circulation of water.
ASCIDIANS
81
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82
N. J. BERRILL
Perophora viridis has a greenish tinge, as its name implies, and
both oozoöid and blastozoöid have four rows of stigmata.
Perophora
listen is similar in that the greenish colour is quite absent, but it
forms by no means such dense colonies.
Perophora annectens differs
from both these species in that it has but three rows of stigmata.
Perophora bermudensis possesses blastozooids
with five rows of
stigmata (although the oozoid has but four) and is accordingly unique.
It may be readily distinguished
from P. viridis with which it is usually
entangled,
by the absence of any green colour and the somewhat
larger size of the zoöids.
Its structure is shown in Fig. 3, and it is seen that not only are
there five rows of stigmata but that about a third of the stigmata of
FIG.
3.
A,
blastozoöid
of
Perophora
bermudensis
n.sp.
B,
oozoöid
of
Ectein
ascidia conklini or else a blastozooid of Perophoropsis herdmani Lahille.
the first and second rows are common to both.
in the smallest and youngest
blastozoöids
discernible.
This condition prevails
in which stigmata
are
Perophoropsis herdmani Lahille.
This form was described by Lahille in 1890 from Banyuls and has
not been rediscovered.
It differs from species of Perophora in that
there are at least ten rows of stigmata.
It is a genus therefore inter
mediate between Perophora and Ecteinascidia.
At Bermuda, mixed with colonies of Perophora viri4is, P. bermu
densis, and Ecteinascidia conklini, were found individuals corresponding
in structure to Perophoropsis;
these are shown in Fig. 4.
ASCIDIANS
OF
BERMUDAS
83
There is, however, another possibility.
It was determined in the
case of Ecteinascidia
tunbinata, E. conklini, Perophora bermudensis,
Clavelina lepadiformis, Botryllus gigas, and Botrylloides leachii, that in
developing blastozooids
the number of rows of stigmata, while yet
non-functional
although perforate, equals or may slightly exceed the
number of stigmata rows in the zooid from which they arose.
In no
case was it less than that number.
This statement
is probably true
for ascidians as a whole and will form the subject of further investi
gation.
This phenomenon
accordingly
rules out the possibility that the
Perophoropsis-like
forms were young blastozoöids of Ecteinascidia;
while the fact that the oozoöid of E. turbinata possesses from the start
twelve rows of stigmata
makes it impossible that they are young
oozooids of that species.
With E. conklini the case is different.
The
oozooid first becomes active with but six rows of stigmata, and ac
cordingly it is possible that the forms in question are slightly-grown
individuals of this type, especially as they were solitary and without
budding stolons, although occurring not far from one another.
An
other feature which supports such a contention is that the individual
stigmata
are but
species.
Whether
half
the
they
length
of those
are blastozoöids
of Ecteinascidia
or oozoöids,
or Perophora
they
are probably
but half-grown.
Provisionally,
therefore, these perophorids will be assumed to be
young oozoöids of Ecteinascidia conklini rather than the Perophoropsis
of Lahille.
If this is the case, then it should be noted that additional
rows of stigmata appear in the oozooid when the stigmata of the original
six
rows
are
merely
a fourth
of full
adult
size,
and
cannot
be distin
guished from those first six rows.
Genus Clavelina.
There is some confusion as to the number of species of Clavelina
to be found in the Bermuda and West Indian region.
From material taken on the Challenger expedition at Bermuda
Herdman described a form that he named Clavelina oblonga.
In 1900
Verrill, in his general account of the fauna of the Bermudas, described
an ascidianunder the name Diazona picta. In the account of the
Bermuda ascidians published by Van Name in 1902 this last was
changed to Rhodozona piclaon the ground that itdiffered
materially
from the type of Diazona and was intermediate
in character between
Diazona and Clavelina.
In his later account (1921) of the ascidian
fauna of the West Indies Van Name includes both these forms under
Clavehna oblonga, the Diazona picta of Verrill being considered to
be large colonies and Herdman's
Clavelina oblonga young colonies of
84
N. J. BERRILL
one and the same species.
This he describes as a form in which the
zoöids of a young colony are almost completely separated from one
another but which in older colonies become enclosed for their greater
part in a common gelatinous test.
Such a conclusion is believed to be the result of two factors, the
study mainly of preserved material, and of material collected not later
than May, that is, of material somewhat immature.
Preservation
destroys
pigmentation
and
in these
two
forms
results
shrinkage of the thoracic part of the zoöids.
These forms are believed to be both clavelinids
distinct
species.
On
this
assumption
then
the
in a tremendous
but also to be quite
one
should
be
known
as Clavelina oblonga Herdman and the other as Clavelina picta (Verrill).
It is hoped that the following descriptions will show this to be the case.
Clavelina oblonga Herdman.
Clavelina picta (Verrill).
Habitat.
Habitat.
Attached
to the under-surface
of
stones near low-water
level, very
rarely in more exposed positions.
Attached
to
sea-fans,
corals,
submerged
wrecks, etc., from low-water level to a few
fathoms;
never found under stones; a good
flow of water seems to be necessary.
Breeding season.
Breeding season.
End of April until August.
End of June until August or September.
Form of colony.
Number of zoöidsrarely exceeds 40,
usually
much less. They are at
tached to a basal stolon, but other
wise are separated
from one an
other.
Budding occurs throughout
breeding
season so that
zoöids of all
sizes are to be found.
Pigmentation.
Test
crystal-clear.
Branchial
sac
with flecks of white pigment near its
anterior
end.
Abdominal
region
yellowish.
colony
In living
perfectly
state
transparent.
whole
Form of colony.
Colonies usually possess more than 40 zooids
and may have as many as 1000.
The zoöids
are embedded
in a common
test, which is
usually
divided
into
corms.
During
the
summer
the thorax
of each zooid extends
beyond the common test, at other times it is
usually completely
embedded.
Budding does
not occur during the breeding season or the
months
preceding
it and so the size of the
zoöids is very uniform in any one colony.
Pigmentation.
Test unpigmented but slightly opaque. En
dostyle, dorsal lamina, and peripharyngeal
bands densely coloured with purple or carmine
cells.They alsoextendthroughoutthe ab
domen and then accumulate in the ends of the
test vessels. They appear in the late embryo
in such places and the oozooid is as highly
coloured
Egg and tadpole
as the blastozoöids.
Egg and tadpole size.
size.
0.31mm. diameter(egg).
0.49 mm. diameter (egg).
2.25 mm. total
3.30 mm. total
length
(tadpole).
length
(tadpole).
Post-larval development.
Post-larval development.
Tadpole and young oozooidhave Tadpole and young oozooid
fromthefirst,
two rowsofdefinitive
first, four rows of definitive
stigmata.
other
species
of Clavelina
have from
stigmata.
have but two.
the
All
ASCIDIANS
85
OF BERMUDAS
Clavelina oblonga usually occurs in groups of two or three during
May and increases in number throughout
the breeding season until
about forty zoöids of various size may be found in one colony.
This
was found to be the case in both 1930 and 1931, and the implication
is that the oozoöid forms a few winter statoblasts
which regenerate
and give rise to the type of colony just described during the following
spring, while such a colony dies off at the end of the summer, failing
to form a second generation of statoblasts.
That is, the life-cycle is
completed within 18 months; otherwise, large colonies should be found
similar to those of Cksvelina lepadiformis, which is not the case.
That Clavelina picks was originally taken to be a diazonid is due
C
FIG.
4.
A,
colony
of
Clavelina
oblonga
Herdman.
B,
C,
egg
and
tadpole
of
same. D, partof a colonyof Clavelina
picta(Verrill).
E, F, egg and tadpole
ofsame. G, H, eggand tadpoleof Clavelina
lepadiformis.
The eggs and tadpoles are drawn to the same scale.
to a superficial similarity in type of budding.
In Diazona violacea
budding by abdominal strobilation
occurs after the breeding season
has ended, and only then.
So that every autumn each zooid forms
eight or ten compact bodies which regenerate
slowly through the
winter
and
spring
to
become
sexually
mature
in
the
late
summer;
the process is then repeated.
Therefore the zoöids are uniform in
size and remain embedded in the common test, although the anterior
part of the thorax of each extends separately during the late summer.
In Clavelina picks winter statoblasts
are formed after the breeding
season and apparently at no other time.
The oozooid probably forms
about ten such bodies.
These regenerate, grow and become sexually
86
N. J. BERRILL
mature during the winter and spring.
They carry on these processes
close together within the test of the oozoöid, and so remain within a
common
test.
resulting
in
During
the
the
formation
late
summer
during
the
these
processes
following
year
are
repeated,
of a colony
composed of about ten corms, each containing
ten or more zooids.
This may be repeated a third year to form massive colonies.
During
maturity
the anterior parts of the zoöids extend from the common
test just as in Diazona, and as in Diazona they contract and degenerate
after the breeding season while the thoracic extensions of the test are
sloughed off. Thus, apart from the difference in budding, strobilation
of the abdomen in Diazona and the formation
of post-abdominal
extensions in Clavelina, the cycle is much the same in the two genera
and it is understandable
that a superficial similarity
results.
The
separation of the zoöids in large colonies of Clavelina lepadiformis in
contrast to the condition in C. picks is due probably to the very thin
test of the former, so thin that when the statoblasts
grow into new
individuals they form independent
vertical extensions of it from the
first.
Budding in these forms will be the subject of a much more detailed
investigation.
Svmplegma
vinide Herdman.
This species was described first by Herdman from material collected
at Bermuda by the Challenger expedition,
and has since been dis
covered throughout
the West Indies, and in the East Indies from the
Philippines to the Red Sea. A full description with illustration
of a
zoöidis given by Van Name (1921).
This form has at various times been included in the Styelida@ and
in the Botryllida@.
A few additionalobservationsmade at Bermuda
merely emphasize its relationship to both these groups.
In fact it so
completelybridgesthegap,both initsadultstructure
and organization
and in its development,
between these two families that their main
tenance as distinct
families is an unnatural
classification.
It is
accordingly proposed that the Botryllida@ be subordinated
within the
Styelithe, either as a sub-family or merely as constituent
genera.
The method of budding in its fundamentals
is identical in the
Polystyelida@, Symplegma, and Botryllida@.
Symplegma resembles polystyelids
such as Polyandrocarpa,
Dis
tomus, or Stolonica inasmuch as each zoOid has its own atrial siphon,
common cloacal cavities not being formed.
Its general anatomy
resembles that of Polyandrocanpa
or Polycarpa
except that its in
dividuals
are much smaller.
Correlated
with this reduction
is a
ASCIDIANS
diminution
number
in number
of polycarps
of rows of stigmata.
lids only in the absence
87
OF BERMUDAS
(to
Its structure
of common
cloacal
a single
polycarp)
and
in
differs from that of botryl
cavities,
and consequently
in the lack of obvious systems in the colony.
In other words, common
cloacalcavities
and the arrangement of zooidsintosystems are the
only major features that separate the botryllids from other styelids.
Apart from these two characters Symplegma is so similar to Botryllus
and Botrylloides that it exhibits the same degree of variation in colour
and form of whole colonies.
These varieties are even more definite
and constant in Symplegma than in the Botrylliche, the commoner
A
C
D
B
FIG. 5. A, colony of Symplegma viride stolonica n.y. B, colony of Symplegma
viride typica. C, zoöid of stolonica. D, zoöid of typica. E, F, G, tadpoles of
Symplegma viride, Distomus variolosus, and J3otryllus gigas, respectively, drawn to the
same scale.
colour varieties being,—green,
black and white, and orange.
green and white, green and brown,
With the exception of the last, all
colonies
sheet-like
are very
compact
and
as in botryllids,
the
adult
zooids being pressed together, young buds appearing in large numbers
together with clusters of ampulla@ at the colony margin.
The orange
variety, however, is different.
Instead of the budding stolons being
very short, they are even longer than in Stolonica socialis, with the
result that the zoöids are often widely separated
from each other
while the individual zooids assume a comparatively
vertical position.
It may, in view of this difference, be worth recognizing the orange
88
N. J. BERRILL
form as a definite variety.
If so, it is suggested that it be named
Symplegma viride stolonica.
The essential
unity of such forms as Symplegma, Botryllus,
and
Distomus is perhaps best shown in the study of the development.
The tadpole of Symplegtna is almost identical with those of Dis
tomus, Stolonica, and Styelopsis, except that it is somewhat smaller.
It has the same degree of organization;
it has the same single but
composite sense organ (probably developed from the otolith after the
primitive eye was lost, an otolith and eye being found only in Styela
itself); and in Symplegma and Distomus there is a long anterior mental
process surrounded
by a ring of ampulke.
Botryllus and Botrylloides
both produce tadpoles with a similar mental process and ring of
ampulke, there being eight ampulke in each of these genera and also
in Sympiegma.
While the botryllid
tadpoles are the more highly
organized, the young oozoOids developing from these and from tadpoles
of Symplegma are hardly to be distinguished.
Altogether there are greater differences between a genus such as
Styela and the polystyelids
than there are between these last and the
botryllids.
REFERENCES
LAHILLE,
F.,
1890.
VAN NAME, W. G.,
Recherches
1902.
sur
les Tuniciers
The Ascidians
des côtes de France.
of the Bermuda
Islands.
Toulouse.
Trans.
Conn.
Acad. Arts and Sci., 11: 325.
VAN
NAME,
W.
G.,
44: 283.
1921.
Ascidians
of the
West
Indies.
Bull.
Am.
Mus.
Nat.
Hist.,